The present invention relates to operating point trimming methods for an optical waveguide modulator and an optical waveguide switch.
In a general optical communication system, a light signal intensity modulated on the transmitter side is transmitted to the receiver side through an optical transmission line formed of an optical fiber or the like, and the signal is directly detected and the transmitted information is reproduced on the receiver side. The intensity modulation may be performed on the transmitter side by having the drive current of a light source, which is formed for example of a semiconductor laser, modulated with a modulating signal (direct modulation). In the case of the direct modulation, however, there sometimes occurs a relatively large wavelength shift (chirping) when high speed modulation is performed. Therefore, when an optical fiber whose dispersion characteristic is not good is used as the optical transmission line, the transmission is restricted in the transmission distance or the transmission speed. More specifically, when a system is structured with a combination of a single mode optical fiber exhibiting zero dispersion at 1.3 .mu.m band and a semiconductor laser oscillating at 1.55 .mu.m band minimizing the transmission loss, the system has restriction of the transmission distance or the transmission speed due to the wavelength dispersion. As a means for removing such restriction on the transmission distance or transmission speed, there is an external modulation system using an optical modulator. The external modulation system is a system in which a light beam emitted from a light source at the level of constant intensity is modulated by, for example, an optical waveguide modulator provided independently of the light source and, with which, a system producing an extremely small quantity of wavelength chirping can be formed. In implementing such external modulation system, it is desired that a trimming method for controlling the operating point of the optical waveguide modulator is provided.
FIG. 1 is a plan view showing a structure of a conventional optical waveguide modulator 1.
Referring to FIG. 1, reference numeral 2 denotes a waveguide substrate in a parallelepiped form made from a ferroelectric material such as lithium niobate (LiNbO.sub.3). At a predetermined position on the surface of the waveguide substrate 2, there is formed, by such a method as thermal diffusion of titanium (Ti), an optical waveguide 3 having a first branch optical waveguide 3a and a second branch optical waveguide 3b of the same length and in the form of a combined two Y-branch waveguides. On the surface of the waveguide substrate 2, there is further formed a buffer layer 4 of a silicon dioxide (SiO.sub.2) film for covering all of the optical waveguides 3, 3a, and 3b.
Over the first branch optical waveguide 3a, there is formed, through the buffer layer 4, a first electrode 5a for applying the driving voltage, provided by plating of a conductive material such as gold, and over the second branch optical waveguide 3b, there is formed, through the buffer layer 4, a second electrode 5b. One end of both the first and second electrodes 5a and 5b are connected with a drive circuit (not shown) for changing the voltages applied to the electrodes 5a and 5b according to the modulating signal and the other ends thereof are connected with terminating resistors (not shown). One end portion of the optical waveguide 3 is optically coupled on the waveguide substrate 2 with an optical fiber 6 on the input side as indicated by the arrow and the other end portion thereof is optically coupled with an optical fiber 7 on the output side. The buffer layer 4 is provided so that the light beams propagating through the first and second branch optical waveguides 3a and 3b may not be absorbed by the first and second electrodes 5a and 5b.
With the described arrangement, the refractive indexes of the first and second branch optical waveguides 3a and 3b of the optical waveguide 3 are changed according to the electric fields applied thereto and, hence, the respective branch beams branched by the branch optical waveguides 3a and 3b in phase will suffer phase changes according to the refractive indexes of the respective branch optical waveguides 3a and 3b.
Since, except for the Y-branch portions, the optical waveguide 3 is arranged to be a single mode optical waveguide which propagates only the light beam of the fundamental mode, the intensity of the interference light output from the optical waveguide 3 becomes a maximum when the phase difference between the branch light beams is zero, whereas the intensity of the interference light becomes a minimum when the phase difference between the branch light beams is .pi.. Further, when the phase difference is between 0 and .pi., the interference light takes on an intensity corresponding to the phase difference. In the described manner, the optical waveguide modulator 1 provides time-series changes to the output light intensity by changing the voltages applied to the electrodes according to the modulating signal.
FIG. 2 shows the relationship between the applied voltage and the output light intensity. In the diagram, the output light intensity is indicated along the axis of ordinate and the applied voltage is indicated along the axis of abscissa.
The relationship between the applied voltage V to the Y-branch interferometric optical modulator and the output light intensity P.sub.o, under the conditions that either a TE mode wave or a TM mode wave is used, that the propagating light is equally divided and combined at the Y-branches suffering no loss, and that the wave guide has no loss, is expressed as EQU P.sub.o =Pi.multidot.cos.sup.2 [(.pi./2).multidot.(V/V.pi.)+.theta.],
where Pi represents the input light intensity and V.pi., called the half-wave length voltage, represents the minimum voltage difference providing the maximum output and the minimum output.
The angle .theta. becomes 0 when the optical path difference between the two waveguide paths 3a and 3b equals 0 or .lambda. (wavelength of the propagating light)/n (the refractive index of the medium for the propagating light) multiplied by 2 k (k=.+-.1, .+-.2, . . . ). The solid line 10 in FIG. 2 shows the case where the angle .theta.=0.
In such modulator, the intensity modulation can be achieved by changing the voltage between the applied voltage V.sub.1 (V.sub.3) providing a maximum of the light intensity and the applied voltage V.sub.2 (V.sub.4) providing a minimum of the light intensity. For example, when performing digital modulation, the applied voltage may be set to V.sub.1 at the mark period 10a corresponding to "1" and the applied voltage may be set to V.sub.2 at the space period 10b corresponding to "0".
In the described optical waveguide modulator 1, the refractive indexes of the first and second branch optical waveguides 3a and 3b are changed by stress exerted thereon from the buffer layer 4 and the electrodes 5a and 5b disposed on the waveguide substrate 2 and, as a result, the optical path lengths of these waveguides become different, leading to a shift of the operating point.
When the operating point is shifted as described above, the desired waveform 10 is shifted to the position of the waveform 11 as indicted in FIG. 2 by the broken line. More specifically, the waveform is shifted by the shift amount .delta.V corresponding to the operating point shifting. In order to compensate for this shift, it is required that a DC bias voltage is applied to the electrodes 5a and 5b to change the refractive indexes of the branch optical waveguides 3a and 3b and adjust the optical path difference therebetween.
The shift amount .delta.V due to the operating point shifting differs from product to product because of lack of uniformity of the thickness of the evaporation film of the buffer layer etc. and of the size of various parts occurring in the course of fabrication. Such tendency is especially remarkable with optical waveguide modulators using, as the waveguide substrate, LiNbO.sub.3 having the photoelastic effect. Therefore, it becomes necessary to apply each optical modulator with the DC bias voltage corresponding to the operating point shift amount .delta.V due to the operating point shifting resulted from the above described lack of uniformity to thereby adjust the DC offset. Further, even if the shift amount .delta.V is compensated for by taking the above measure, there sometimes occurs, when such DC bias voltage is applied, a phenomenon called DC drift in which the operating point shifts with the lapse of time after the application of the DC bias voltage. This phenomenon for example is such, as shown in FIG. 3, that the waveform 10 adjusted to a desired output light intensity by application of a DC bias tends to shift to the waveform 12 indicated by the broken line.
When such a shift occurs, the applied voltage V.sub.1 providing the maximum light output is shifted to V.sub.1 ', as shown in FIG. 3, while the applied voltage V.sub.2 providing the minimum light output is shifted to V.sub.2 '. Therefore, if the modulation is performed with the applied voltages V.sub.1 -V.sub.2 whereby the original waveform 10 was obtained, the output light intensity when the applied voltage is V.sub.1 decreases from 10a to 10b as shown in the same diagram, while the output light intensity 12a when the applied voltage is V.sub.2 increases from 12a to 12b. As a result, it becomes impossible to obtain the maximum and minimum output light beams and the extinction ration (the ratio of the output light intensity in the mark period to that in the space period given in decibels) is decreased, and a problem is presented that it becomes unable to have a suitable optical signal output.
When it is attempted to obtain optical waveguide modulators having no operating point shifting such as the DC drift, since some nonuniformity of the operating point occurs in the fabrication process, it becomes necessary to select such products that have virtually equal operating point shiftings after fabrication thereof. This is indeed a difficult task in view of the fact that the nonuniformity of the operating point shifting is frequently produced in the fabrication process, and because of which, a problem is presented of a lowering of the yield rate.